"Dangerous ocean reef predators." This phrase conjures up torpedo-shaped gray bodies, sharp dorsal fins, and even sharper teeth. Yet sharks are not the most dangerous animals on the reef, for humans. Most reef sharks aren't very aggressive, although they can cause injuries if they feel threatened. In fact, a surprising array of smaller animals is much more dangerous to human reef visitors.
The coral reefs are frequented by a selection of mostly small-to-midsized sharks, including nurse sharks, lemon sharks, black-tip reef sharks, white-tip reef sharks, and zebra sharks. All of these animals will give a diver a nip if they are cornered, but none have the limb-severing capabilities of a great white. The only seriously scary shark that a person might encounter on the reef is the tiger shark. Although they're ordinarily an open-water predator, tiger sharks sometimes pursue prey into the reef, where they could come into conflict with humans. Tiger sharks have a reputation for aggression, and are second only to great whites in serious attacks on humans.
While many predatory fish roam the reefs, they are not known for seriously injuring humans. Such animals as barracuda, moray eels, and giant trevally look toothy and potentially dangerous, but, as with reef sharks, don't bite people unless they feel threatened.
The fish to watch out for are those with poisonous fins. These include members of the scorpion fish family, such as the lion fish, and bottom-dwellers like stone fish. Lion fish have a row of venomous spines along both their dorsal fins and their enlarged pectoral fins. While they haven't been implicated in human deaths, lion fish stings do cause severe pain, shock, paralysis, and necrosis (tissue death). Stone fish stings are even worse. They cause all of the above symptoms, plus cardiac arrest and possible death. Stone fish tend to rest on the bottom near reefs, where an unwary diver might step on them. Their brown bodies blend in perfectly with the sand.
All fifteen species of sea snake that are found on the reef are armed with deadly venom. These animals tend to be very curious, and will often swim right up to a scuba diver or snorkeler to inspect them. Sea snakes are not aggressive, however, and have very small fangs. They have not caused any reported deaths in humans.
Blue Ringed Octopus
These tiny, beautiful cephalopods are extremely dangerous, in part because they look so harmless and cute. The blue ringed octopus is only about the size of a hen's egg, but its poison can kill a person in minutes. There is no antidote. Any diver who sees a tiny octopus with glowing electric blue rings had best back away quickly; the electric blue signals that the animal feels threatened. Oh, and the octopus' beak can bite through a wetsuit!
Two types of cnidarians, the box jelly and the irukandji jelly, are among the most deadly creatures encountered on the reef. Box jellies have been blamed for at least 5,567 human deaths since 1884, according to Wikipedia. Their smaller cousins, the irukandjis, have killed at least 2 people in Australia. A diver can wear a very fine mesh material such as nylon pantyhose over any exposed skin for protection against these filmy menaces, but a single tentacle hitting a small gap in the material can trigger a fatal reaction. It's probably best just to stay out of the water in northern Australia between November and May.
Dangerous predatory creatures do patrol the world's coral reef ecosystems but they may not be the animals that spring to mind. Who would have guessed that jellyfish smaller than your fist are more likely to harm a diver than sharks are?